In recent years, with the spread of a network such as the internet, the traffic volume of a backbone network is increasing rapidly. With this change, an ultra high speed optical communication system like 100 Gbps is desired. As to the technology which realizes the ultra high speed optical communication system, an optical phase modulation method and a polarization multiplex and demultiplex technology have been watched with interest.
Unlike a conventional light intensity modulation method which performs data modulation to the light intensity of the transmission laser beam, the optical phase modulation method is a method to perform data modulation to the phase of the transmission laser beam. As for the optical phase modulation method, QPSK (Quadrature Phase Shift Keying), 8PSK (8 Phase Shift Keying), and QAM (Quadrature Amplitude Modulation) and so on are known. In the optical phase modulation method adopting these data modulation methods, symbol rate (baud rate) can be decreased by assigning a plurality of bits to one symbol. For this reason, the optical phase modulation method can reduce operation speed of an electric device. Also, as a result, in the optical phase modulation method, reduction in production cost of the apparatus can be expected. For example, when QPSK is used, 2 bits (for example, 00, 01, 11, 10) are assigned respectively to four of optical phases (for example, 45 degrees, 135 degrees, 225 degrees, 315 degrees). For this reason, the symbol rate of QPSK can be reduced to 1/2 of symbol rate (that is, bit rate) of the light intensity modulation method.
In order to receive signal light modulated by the optical phase modulation, an optical coherent method is used. This is a method which combines the signal light with the laser beam having a frequency almost the same as the signal light (which is called local oscillation light) by an optical element called a 90-degree hybrid and thereby receives the output by a photoelectric detector. Here, for the sake of making description simple, it is supposed that polarization states of the signal light and the local oscillation light are the same linear polarization. When the optical coherent method is used, an alternating current component of an electric signal outputted from the photoelectric detector is a beat signal of the signal light and the local oscillation light. And, amplitude of the beat signal is proportional to the light intensity of the signal light and the local oscillation light. And, phase of the beat signal will be the phase difference between the signal light and the local oscillation light when a carrier wave frequency of the signal light and a frequency of the local oscillation light are identical. When the phase of the local oscillation light is identical to the phase of the laser beam inputted to an optical modulator in a transmission end, the phase of the beat signal is the phase given to the laser beam at the transmission end. For this reason, it is possible to demodulate transmission data by converting the phase of the beat signal into a bit string using symbol mapping.
Actually, the values of the carrier wave frequency of the signal light and the frequency of the local oscillation light are not identical perfectly. Further, the phase of the local oscillation light in an optical receiver and the phase of the laser beam inputted to the optical modulator in an optical transmitter are not identical either. Accordingly, it is necessary to compensate the influence caused by the optical phase deviation that is the phase difference between the signal light inputted to the optical modulator in the optical transmitter and the local oscillation light. Further, it is necessary to compensate the influence caused by the optical carrier wave frequency deviation that is the difference between the carrier wave frequency of the signal light in the optical transmitter and the frequency of the local oscillation light. However, as the specific method for compensating the optical phase deviation and the optical carrier wave frequency deviation is not particularly required for describing the present invention, the description will be omitted.
On the other hand, the polarization multiplex and demultiplex technology has also been watched with interest as one of the technologies to realize the ultra high speed optical communication system. In the polarization multiplex and demultiplex technology, the optical transmitter multiplexes two independent optical signals, which have carrier waves deployed in an identical frequency band and the polarization states are orthogonal to each other, and thereby transmits. Further, the optical receiver separates the above-mentioned two independent optical signals from a received signal. As a result, the polarization multiplex and demultiplex technology realizes double transmission rates. In other words, it is possible to reduce operation speed of an electric device and thereby can reduce the apparatus cost because the polarization multiplex and demultiplex technology can make symbol rate (baud rate) of an optical signal 1/2.
By combining both of the optical phase modulation method and the polarization multiplex and demultiplex technology mentioned above, the ultra high speed optical communication system like 100 Gbps can be realized. The process to compensate the optical carrier wave frequency deviation and the optical phase deviation, and the process to separate to two independent optical signals (polarization separating process) can be realized by a digital signal processing circuit implemented by LSI (Large Scale Integration) or the like. The technology which demodulates transmission data with a high degree of accuracy using the digital signal processing circuit like this is called an optical digital coherent communication system.
In the following, receiving process in the ultra high speed optical communication system using the optical digital coherent communication system will be described with reference to drawings.
FIG. 4 is a block diagram showing a configuration of an optical receiver 90 of related art in the optical communication system using the optical digital coherent communication system. The optical receiver 90 inputs a received optical signal from an optical transmission path to a 90-degree hybrid 91 together with the local oscillation light having a frequency almost the same as the carrier wave frequency of the received optical signal. The 90-degree hybrid 91 outputs total of four optical signals which are real part components and imaginary part components of optical signals having the polarization states parallel to each of two orthogonal polarization axes. These four optical signals, after having been converted into analog electric signals by photoelectric detectors 92a-92d, are converted into digital electric signals by analog digital converters (hereinafter, described as ADC) 93a-93d. These digital electric signals, after having been converted into the digital electric signals sampled with symbol rate (baud rate) of the received optical signal by a re-sampling unit, which is not illustrated, are inputted to a polarization separation device 94. The polarization separation device 94, based on four inputted digital electric signals, extracts the electric signals corresponding to two independent optical signals being polarization multiplexed. With respect to each of the extracted electric signals, optical phase rotation caused by the optical carrier wave frequency deviation and the optical phase deviation between the received optical signal and the local oscillation light is compensated by optical carrier wave frequency deviation and optical phase deviation compensation units 95a-95b respectively. After that, each of the electric signals is demodulated to the original transmission bit string by symbol discrimination units 96a to 96b respectively.
As described above, this optical receiver 90 of the related art operates as follows. That is, with combining the optical phase modulation method and the polarization multiplex and demultiplex technology, the optical receiver 90 compensates the influence to each of the electric signals corresponding to two independent optical signals separated by polarization separation by the optical carrier wave frequency deviation and the optical phase deviation. As a result, the optical receiver 90 of the related art can realize the ultra high speed optical communication system like 100 Gbps.
Next, the polarization separation device 94 provided in the optical receiver 90 of the related art will be described.
FIG. 5 is a block diagram showing a configuration of the polarization separation device 94. As shown in FIG. 5, the polarization separation device 94 includes a filter units 901a-901d and filter coefficient update units 902a-902b. Further, in FIG. 5, an input signal 1 is an electric signal corresponding to the optical signal having the polarization state parallel to one of two orthogonal polarization axes in the 90-degree hybrid 91 in FIG. 4. That is, this input signal 1 is represented by a complex number which includes the digital electric signal outputted from ADC 93a of FIG. 4 as a real part component and the digital electric signal outputted from ADC 93b as an imaginary part component.
Similarly, an input signal 2 of FIG. 5 is an electric signal corresponding to the optical signal having the polarization state parallel to the other of two orthogonal polarization axes in the 90-degree hybrid 91 in FIG. 4. That is, this input signal 2 is represented by a complex number which includes the digital electric signal outputted from ADC 93c of FIG. 4 as a real part component and the digital electric signal outputted from ADC 93d as an imaginary part component.
An output signal 1 and an output signal 2 of FIG. 5 are signals which will be regenerated as the electric signals corresponding to two independent optical signals having been polarization multiplexed in the optical transmitter respectively.
The filter units 901a-901d of FIG. 5 perform filtering process of the input signal 1 and the input signal 2 respectively using filter coefficients set to each of the filter units independently. After that, the summation of the filter unit 901a and the filter unit 901c is outputted as the output signal 1. The summation of the filter unit 901b and the filter unit 901d is outputted as the output signal 2. Further, as the filter units 901a-901d, general FIR (Finite Impulse Response) filters can be used.
The filter coefficient update unit 902a updates the filter coefficients of the filter units 901a and 901c according to predetermined algorithm. Similarly, the filter coefficient update unit 902b updates the filter coefficients of the filter units 901b and 901d. As the algorithm for the filter coefficient update units 902a-902b to update each of the filter coefficients, CMA (Constant Modulus Algorithm) is widely used. CMA is the algorithm which controls each of the filter coefficients of the filter units 901a-901d adaptively so that an envelope of the extracted electric signal may become fixed, that is, the strength may become fixed, and thereby performs polarization separation. Additionally, as the algorithm which updates the filter coefficients of each of the filter units, the LMS (Least Mean Square) algorithm is also general (refer to non-patent document 1), however, an example using CMA will be described here.
An example of error functions defined in CMA are indicated in the following formula (1).Jx(W,WH)=E[(rx2−|E′x|2)2],Jy(W,WH)=E[(ry2−|E′y|2)2]  formula (1)In formula (1), Jx(W,WH) is an error function of the output signal 1, and Jy(W,WH) is an error function of the output signal 2.
Here, W is a matrix which is called Jones matrix of 2×2 of the size representing the inverse characteristics of an optical transmission path. Component-11 (Wxx), component-12 (Wxy), component-21 (Wyx) and component-22 (Wyy) of matrix W are respective filter coefficients of each of the filter units 901a-901d. Matrix WH is the Hermitian conjugate of matrix W.
Further, here, although the number of taps of the filter unit is supposed to be 1 for the sake of making description simple, the number of taps may be 2 or more. Further, rx and ry are respective target values of amplitude of the output signal 1 and the output signal 2. Also, Ex′ and Ey′ are amplitude of the output signal 1 and the output signal 2 respectively. Further, E [x] represents an expectation value of x.
The filter coefficient update unit 902a updates each of the filter coefficients of the filter units 901a and 901c sequentially so that Jx may become the smallest. Also, the filter coefficient update unit 902b updates each of the filter coefficients of the filter units 901b and 901d sequentially so that Jy may become the smallest.
An update formula for the filter coefficient update units 902a and 902b to update the filter coefficient based on the error functions of CMA of formula (1) is indicated in the following formula (2).
                              W                      k            +            1                    H                =                                            W              k              H                        +                          μ              ⁢                              ∇                                  J                  ⁡                                      (                                          W                      ,                                              W                        H                                                              )                                                                                =                                                    W                k                H                            +                              μ                ⁢                                                      ∂                                          J                      ⁡                                              (                                                  W                          ,                                                      W                            H                                                                          )                                                                                                  ∂                    W                                                                        =                                          W                k                H                            +                              μ                ⁡                                  (                                                                                                                                          ∂                            J                                                                                ∂                                                          w                              xx                                                                                                                                                                                                        ∂                            J                                                                                ∂                                                          w                              xy                                                                                                                                                                                                                                                ∂                            J                                                                                ∂                                                          w                              yx                                                                                                                                                                                                        ∂                            J                                                                                ∂                                                          w                              yy                                                                                                                                                            )                                                                                        formula        ⁢                                  ⁢                  (          2          )                                                  ∇                      J            ⁡                          (                              W                ,                                  W                  H                                            )                                      =                                            ∂              J                                      ∂              W                                =                                    (                                                                                                                  ∂                        J                                                                    ∂                                                  w                          xx                          *                                                                                                                                                                        ∂                        J                                                                    ∂                                                  w                          yx                          *                                                                                                                                                                                                        ∂                        J                                                                    ∂                                                  w                          xy                          *                                                                                                                                                                        ∂                        J                                                                    ∂                                                  w                          yy                          *                                                                                                                                )                        =                                          -                2                            ⁢                              E                ⁡                                  [                                      (                                                                                                                        e                            xx                                                                                                                                e                            xy                                                                                                                                                                            e                            yx                                                                                                                                e                            yy                                                                                                                )                                    ]                                                                                                                                                  ⁢                              e            xx                    =                                    (                                                r                  x                  2                                -                                                                                                E                      x                      ′                                                                            2                                            )                        ⁢                          E              x                        ⁢                          E              x                              ′                *                                                                                                                                  ⁢                              e            xy                    =                                    (                                                r                  x                  2                                -                                                                                                E                      x                      ′                                                                            2                                            )                        ⁢                          E              y                        ⁢                          E              x                              ′                *                                                                                                                                  ⁢                              e            yx                    =                                    (                                                r                  y                  2                                -                                                                                                E                      y                      ′                                                                            2                                            )                        ⁢                          E              x                        ⁢                          E              y                              ′                *                                                                                                                                  ⁢                              e            yy                    =                                    (                                                r                  y                  2                                -                                                                                                E                      y                      ′                                                                            2                                            )                        ⁢                          E              y                        ⁢                          E              y                              ′                *                                                                                    Here, μ is a parameter to stabilize feedback control by adjusting the update amount of the filter coefficient. In calculation of the update amount of the filter coefficient, it is general to substitute an instantaneous value for an expectation value.
Formula (1) is the error functions that are generally used for polarization separation of a polarization multiplexed QPSK signal, and Jx will be 0 when Ex is on a circle with the radius of rx, and similarly, Jy will be 0 when Ey is on a circle with the radius of ry. Because the condition of the error functions Jx and Jy to be 0 is a necessary and sufficient condition for polarization separation to be succeeded, polarization separation is enabled by CMA to update the filter coefficients so that the error functions Jx and Jy will be 0.
Further, other than indicated in formula (1), an error function such that Ex or Ey will be 0 when Ex or Ey exists in any of four symbols of QPSK is also proposed, and is called decision-directed method. Although using formula (1) which is easy for implementing is general because this method is substantially the same as formula (1) with respect to the error function of QPSK, this method has a merit that the method is applicable also to a phase modulation method for a large multiple-value number such as 16QAM to which formula (1) is inapplicable.
As described above, by operation of the filter units 901a-901d and the filter coefficient update units 902a and 902b using CMA of the polarization separation device 94, it becomes possible to separate and extract the electric signals corresponding to two independent optical signals from a received optical signal. As for such filter coefficient updating process in a polarization separation device is also disclosed in patent document 1. Further, another apparatus which performs polarization separating process based on each of signals to be regenerated is also disclosed in non-patent document 2.